1. Field of the Invention
The present invention relates to a webbing driving device for use in a seat belt apparatus which drives webbing for restraining an occupant through a wire so as to automatically fastening the webbing on the occupant, or to release the webbing from the occupant.
2. Description of the Related Art
There are conventional automatic seat belt apparatuses for automatically fastening a webbing on an occupant by driving the webbing in the lengthwise direction of the vehicle using a webbing driving device after the occupant gets in a vehicle are known.
Webbing driving devices of a type which drives the webbing in the lengthwise direction through a wire have been proposed. Such webbing driving devices include a drum and a screw plate to which the wire is connected, and the screw plate is supported by a screw means in such a manner as to be movable in an axial direction of the drum. The webbing driving device of this type is adapted to drive the webbing in the lengthwise direction of the vehicle by rotating the drum and the screw plate and thereby winding the wire around the drum in helical fashion, and by rotating the drum and the screw plate and thereby drawing the wire from the drum while limiting the movement of the wire wound around the drum in the axial direction of the drum by a sheet and the screw plate.
In the webbing driving device of the above-described type, although the wire is wound around the drum in helical fashion, a sheet is flat. This makes the wire wound around the drum by an angle with respect to the sheet and thereby causes a gap or a space between the wire and the sheet fluctuate in the circumferential direction of the drum. Further, the screw plate is also flat relative to the wire which is wound around the drum in helical fashion, and the gap between the wire and the sheet cannot therefore be kept constant. If the gap between the wire and the sheet is too large, noises are generated due to unnecessary movement of the wire which occurs when the wire is drawn from the drum. With a too small gap, the sheet pinches the wire between the wire and the sheet. It is therefore preferable to provide the gap with a fixed value around the periphery of the drum.